Effects of tidally enhanced stellar wind on the horizontal branch morphology of globular clusters

TL;DR

This study explores how tidally enhanced stellar winds in binary systems influence the horizontal branch morphology of globular clusters, showing that such winds can produce diverse HB types and match observed cluster features.

Contribution

It introduces the effect of tidally enhanced stellar winds in binary systems as a key factor shaping HB morphology, providing a new explanation for observed variations.

Findings

Red, blue, and extreme HB stars can be produced without additional assumptions.

HB morphology is insensitive to the tidal enhancement parameter Bw.

Theoretical HB morphology matches observations of NGC 2808, except for the number of blue HB stars.

Abstract

Metallicity is the first parameter to influence the horizontal branch (HB) morphology of globular clusters (GCs). It has been found, however, that some other parameters may also play an important role in affecting the morphology. While the nature of these important parameters remains unclear, they are believed to be likely correlated with wind mass-loss of red giants, since this mass loss determines their subsequent locations on the HB. Unfortunately, the mass loss during the red giant stages of the stellar evolution is poorly understood at present. The stellar winds of red giants may be tidally enhanced by companion stars if they are in binary systems. We investigate evolutionary consequences of red giants in binaries by including tidally enhanced stellar winds, and examine the effects on the HB morphology of GCs. We find that red, blue, and extreme horizontal branch stars are all produced under the effects of tidally enhanced stellar wind without any additional assumptions on the mass-loss dispersion. Furthermore, the horizontal branch morphology is found to be insensitive to the tidal enhancement parameter, Bw. We compare our theoretical results with the observed horizontal branch morphology of globular cluster NGC 2808, and find that the basic morphology of the horizontal branch can be well reproduced. The number of blue horizontal branch stars in our calculations, however, is lower than that of NGC 2808.